Rear-contact rear-junction Si solar cells or so-called “interdigitated-back-contact (IBC)” Si solar cells (hereinafter “rear-contact Si solar cells”) put both front and rear contact on the rear of the cell. Rear-contact Si solar cells offer numerous advantages over conventional front-contact solar cells, including the elimination of optical loss resulting from the solar cell being subjected to shade, and exhibit significant improvement in current output and a higher packing density in module fabrication as compared to conventional front-contact Si solar cells.
Conventional high-efficiency rear-contact Si solar cells are commercial off-the-shelf products manufactured using n- or p-type mono- or multicrystalline Si wafers having a wafer thickness of approximately 200 μm as a starting material, resulting an inflexible rear-contact Si solar cell having a total thickness of about 165 μm.
Although such rear-contact Si solar cells provide significant advantages compared to front-contact Si solar cells, commercially available rear-contact Si solar cells exhibit a limited flexibility in bend radius, as measured by the inside curvature of the cell when it is flexed. The bend radius is the minimum radius one can bend a wafer without it being ruptured by the applied stress. The smaller the bend radius, the greater is the material flexibility and bendability.
The limited flexibility of conventional solar cells is due to the use of thick Si wafers having a thickness of about 200 μm as starting materials. Although inherently ultra-thin Si wafers exhibiting full flexibility can be employed as starting materials for fabrication of flexible solar cells, processing and handling of such ultra-thin silicon wafers are troublesome because of their fragile material nature, especially for wafer-level implementation of commercially available solar cell manufacturing with multiple processing steps. In addition, use of ultra-thin Si wafers as starting materials for manufacturing rear-contact, rear-junction solar cells is even more challenging particularly due to complex and multiple processing steps for rear-contact and rear junction formation in making rear-contact Si solar cells such as diffusion processes at high temperature, multiple deposition of thin-films, and the need for very complex and multiple photolithography steps.
Commercially available rear-contact Si solar cells typically are manufactured using n-type Si wafers with a wafer thickness of approximately 200 μm as a starting material. Multiple photolithography steps or printing techniques are used in selectively forming mask for use in etching of silicon dioxide and diffusing dopant and in forming metal contacts in diffused region on the back side of n-type Si wafer.
A key feature of rear-contact Si solar cells is that both p-doped and n-doped regions are alternatively formed in a back surface of the wafer through use of multiple masking and chemical etching technique and diffusion processes. Metal contacts are then made to the p- and n-regions on the backside by forming a seed layer stack, followed by thickening a seed layer by plating ≥20-μm copper and ≥7-μm tin. These interdigitated lines of thick back metals provide a robust foundation for the cell. In a front-side of rear-contact solar cells, the front-surface is chemical etched to form random-pyramids textured Si surfaces, then n-doped region is formed on the textured surface. A dielectric layer stack is then deposited to form anti-reflection coating (ARC) layers. A total thickness of a completed conventional rear-contact solar cell is about 165 μm, with the cell exhibiting a limited bending radius in the range from 50 mm to 60 mm.
Commercial off-the-shelf inflexible rear-contact Si solar cells can be converted to ultra-thin and fully flexible rear-contact Si solar cells via wafer thinning when a thickness of an active Si layer becomes less than 20 μm and thus the bend radius is in the range of 10 mm to 20 mm. This method requires adequate temporary bonding-debonding techniques to achieve cell thinning with no breakage and no damage. However, because of the extensive bumped features exhibited by the back sides of commercial off-the-shelf rear-contact solar cells due to thick interdigitated-back-contact lines, the mechanical properties and electrical performance of such wafer-thinned cells are significantly affected by the choice of the temporary bonding-debonding methods used and the quality of the bonded fixture during the thinning process. When the total thickness variation (TTV) after thinning process is fairly large (e.g., more than 10 μm), it significantly degrades the mechanical strength of thinned solar cell. In addition, generation of micro-cracks on both the front and rear sides of the solar cell during the thinning process is one of the major causes of breakage. Under an applied load, the concentrated stress at the defect degrades the mechanical strength of the thinned solar cell, eventually causing it to fracture. Such defects on the front side can be mitigated in some extent by employing chemical mechanical polishing after thinning, but the process is not cost-effective for use in conventional low-cost solar cell fabrication.
Temporary bonding wax that can be applied onto a thick wafer or a thin-film format is a widely used technique for wafer thinning. However, the bonding wax technique requires an additional step for spin-coating wax or delicate lamination requiring lamination tools and sometimes requires an aggressive solvent for removal of wax residue. Adhesive tape-based temporary bonding-debonding techniques can simplify application of temporary bonding adhesive, dramatically improving thinning process productivity without a spin-coater or laminator. Generally, temporary bonding adhesive tape is supported on polymer films, such as poly-(ethylene terephthalate) (PET), polyimide (PI), polystyrene (PS) or liquid crystal polymer (LCP). On the top of the polymer film, a compression layer with the thickness between 150 μm and 300 μm provides a conformable compression for highly bumped surfaces, followed by a 100-μm thick adhesive. Temporary bonding adhesive tape can be released depending of the release mechanism, such as, UV exposed release, controlled peel release or heat curing release. For low-cost and large arear solar cell application, an adhesive tape based on heat-curing release provides damage-free, fast release of solar cell from temporary boding adhesive tape with no or minimal adhesive residues. The adhesive layer on the top surface are protected by a protective release liner, which need to be removed prior to applying the adhesive tape onto the substrate.